Sometimes Apache hangs (FastCGI + APC)

I’m trying to tuning a VPS with WHM for a single website.
Sometimes, specially when I have more than 50 users connected, Apache hangs: I get a lot of "W" Sending Reply status from WHM » Home » Server Status » Apache Status and after some minutes the site get to work again.
During this problematic status CPU are not loaded and memory isn’t full.
I’ve captured Apache status and mysqlmymonlite report during the problematic minutes (attached) and here some relevant settings: